In the past, there have been proposed various power supply apparatuses which simultaneously operate a plurality of power devices to supply DC power from the power devices to one or more load devices connected to the power devices.
For example, there is the power supply apparatus including all the power devices which make a constant voltage control. In this power supply apparatus, all the power devices are configured to output the same output voltage.
However, in a practical sense, it is difficult to adjust accurately the output voltages of all the power devices to be the same. Therefore, the power supply apparatus is likely to have a difference between the output voltages of the power devices. Consequently, in the aforementioned power supply apparatus, only the power device having the highest output voltage supplies a DC power to the load device in accordance with its available power capacity. In this situation, when the power device having the highest output voltage fails to supply enough power to the load device, the remaining power devices compensate for a shortage of power supply. Thus, in this power supply apparatus, the power device having the highest output voltage, that is, the particular power device is intensively used. Therefore, an advantage obtained from operating simultaneously the plurality of the power devices is reduced.
In order to solve the above problem, there has been proposed a power supply apparatus including two power devices which decrease monotonically its output voltage with an increase of its output current (see Japanese patent laid-open publication No. 10-248253). In this power supply apparatus, the two power devices shows individual output current-output voltage characteristics of which lines have different gradient from each other. This means that, when the two power devices varies their output current by the same extent, one of the power devices shows a variation of the output voltage different from that of the other power device.
In this power supply apparatus, each of the power devices operates to reach a balance point determined by its output current-output voltage characteristics and the load current in accordance with a magnitude of a consumed current (load current) of the load device. Therefore, each of the power devices can output the desired output voltage and output current. Besides, each power device decreases or increases an input voltage (source voltage) by use of a DC/DC converter incorporated therein, thereby generating an output voltage.
The power source connected to the aforementioned power device may be a secondary cell “B” as shown in (a) of FIG. 11. The secondary cell “B” has an internal resistance “r” considered to be connected in series with an ideal voltage source “E” developing electromotive force. Therefore, power loss caused by the internal resistance “r” is increased with an increase of a current (output current) flowing through the internal resistance “r”. Therefore, as shown in (b) of FIG. 11, the secondary cell “B” has a characteristic line indicating that an efficiency η1 (a proportion of output power of the secondary cell “B” to the sum of the output power of the secondary cell “B” and the power loss caused by the internal resistance “r”) is decreased with an increase of the output current of the secondary battery “B”.
The power device “A” connected to the secondary cell “B” has internal loss such as conduction loss (loss caused by an on-resistance of a switching element, a parasitic resistance of an inductor, or the like) in a DC/DC converter incorporated therein. Therefore, the power device “A” has a characteristic line with regard to an efficiency η2 as shown in (c) of FIG. 11. The efficiency η2 is defined as a proportion of output power of the power device to input power of the power device. The input power of the power device “A” is defined as the sum of the output power of the power device “A” and the internal loss of the power device “A”.
As apparent from (b) and (c) in FIG. 11, with regard to a combination of the secondary cell “B” and the power device “A”, an efficiency η3 (a proportion of the output power of the second power device 4c to the sum of the output power of the secondary cell 162 and the loss caused by the internal resistance “r”) varies with the output current of the power device “A”, as shown in (d) of FIG. 11. According to the characteristic line regarding the efficiency η3 shown in (d) of FIG. 11, the efficiency η3 becomes a maximum at a specific output current (output current of the power device “A”). Therefore, with adjusting the output current of the power device “A” connected to the secondary cell “B” to the output current corresponding to the maximal efficiency η3, it is possible to drive the power device “A” efficiently.
However, in the prior power supply apparatus, each power device varies its output current depending on a magnitude of the load current. Accordingly, as shown in (a) of FIG. 11, when the secondary cell “B” is connected to the power device “A” as a power source, the prior power supply apparatus is likely to operate the power device at an insufficient efficiency with regard to the combination of the secondary cell “B” and the power device “A”.